ci(test-core): typecheck test files in CI

[vilenarios]

What

Adds a typecheck script (tsc --noEmit -p tsconfig.json) and runs it in test-core.yml between lint:check and test:ci.

Why

Test code currently has no type gate. Two facts combine into a blind spot:

• yarn build typechecks with tsconfig.prod.json, which excludes **/*.test.ts.
• yarn test runs through a type-stripping loader (no typecheck).

So a type error in a .test.ts — e.g. a mock object missing a newly-required interface field — passes every PR check and only surfaces if someone runs the test-inclusive tsc by hand. This was hit for real while developing the chunk-fanout work: extending BroadcastChunkResult silently broke 5 typed mocks in rebroadcasting-chunk-source.test.ts, green across all of CI.

Scope

Pure gate addition — the current tree already passes yarn typecheck with 0 errors, so there's no cleanup tail. yarn build still owns emit; this is a fast no-emit check that simply includes tests.

🤖 Generated with Claude Code

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 81a094bd-9d10-4983-8bef-a456d8f56283

📥 Commits

Reviewing files that changed from the base of the PR and between feefd25 and 41f5e85.

📒 Files selected for processing (2)

• .github/workflows/test-core.yml
• package.json

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📝 Walkthrough

Walkthrough

This PR adds a typecheck script to package.json that runs tsc --noEmit -p tsconfig.json, and adds a new CI step in the test-core workflow that invokes it after yarn lint:check, with comments explaining why type errors in test files aren't otherwise caught.

Changes

CI Typecheck Addition

Layer / File(s)	Summary
Add typecheck script and wire into CI package.json, .github/workflows/test-core.yml	Adds a typecheck npm script using tsc --noEmit -p tsconfig.json and a new CI step that runs it after lint check, with comments explaining that build excludes test files and test:ci uses a type-stripping loader.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the main change: adding typechecking for test files in CI.
Description check	✅ Passed	The description directly matches the implemented CI typecheck script and workflow change.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/typecheck-tests-ci-gate

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (96ede32) to head (41f5e85).
⚠️ Report is 19 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #809      +/-   ##
===========================================
+ Coverage    78.65%   78.77%   +0.12%     
===========================================
  Files          132      132              
  Lines        49980    50093     +113     
  Branches      3763     3781      +18     
===========================================
+ Hits         39312    39461     +149     
+ Misses       10620    10584      -36     
  Partials        48       48              

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